1. Technical Field
This invention relates to the transmission of widescreen television signals for reception and display on both widescreen televisions and non-widescreen televisions. The term "widescreen television" refers to a television having a display whose ratio of width to height (the aspect ratio) is greater than a predetermined reference value.
One aspect of this invention allows the widescreen transmission to be displayed either in its entirety on a widescreen display or in a contiguous portion on a non-widescreen receiver. Another aspect of this invention relates to the inverse operation of allowing a non-widescreen transmission to be displayed on a widescreen display wherein the aspect ratio of the displayed picture is that of a non-widescreen display.
2. Background Information
The current standard of all television broadcasts has an aspect ratio (the ratio of the display width to the display height) of 4:3, or 1.3333. This aspect ratio was based on the motion picture practice at the time of standardization.
In the United States, Canada and Japan, color broadcasts are made according to the National Television System Committee (NTSC) composite standards. Color video signals broadcast under the NTSC standard require that picture information be separated into two components: luminance, or brightness, and chrominance, or color. FIG. 1 is an amplitude-vs.-frequency diagram illustrating, in simplified form, a typical NTSC composite color television signal 50 comprising a luminance signal 52 and a chrominance signal 54. (A composite television signal is one in which chrominance information is carried on a subcarrier.) The signal occupies a nominal bandwidth of 6 MHz with the picture carrier 56 being 1.25 MHz above the lower end of the band. Luminance information is modulated directly onto picture carrier 56, while chrominance information is modulated onto color subcarrier 58 which is in turn used to modulate picture carriers 56. Color subcarrier 58 has a frequency of 3.579545 MHz, a standard established by the NTSC. (Audio information is carried on another subcarrier 40 lying near the upper edge of the band.)
Television signals are produced and displayed as a result of a line scanning process. The picture information is scanned using a progressive series of horizontal lines which are transmitted sequentially in time. The transmitted signal is a continuous analogue of the brightness intensity corresponding to each point of the line. Such a signal is shown in FIG. 2 from which it may be seen that in a series of standard lines, any two adjacent active line periods (periods during which video information is transmitted) are separated by a period in which no video information is transmitted. This latter period is known as the line blanking interval and is introduced to allow the scanning device in the receiver to reset to the line-start position.
In the NTSC standard, the active line period includes one signal which simultaneously represents the instantaneous values of three independent color components. Other composite signals, SECAM, which is used in France, and PAL, which predominates the rest of Europe, have the same basic format as the NTSC standard, including a line-blanking interval and an active line period in each scan.
The region labeled A in FIG. 1 is of particular importance for it represents overlap between the luminance 52 and chrominance 54 signals. Since separation of luminance and chrominance is accomplished by filtering a frequency-division multiplexed signal, overlaps such as A between the two signals lead to several problems. If, upon reception, complete separation between luminance and chrominance is desired, the necessary filtering will cause the loss of some of the information in both signals. On the other hand, if no loss of information can be tolerated, then one must accept interference between the luminance and chrominance signals. Moreover, since the various parts of the NTSC television signals are transmitted at different frequencies, phase shifts occurring during transmission will affect them differently, causing the signal to deteriorate. Also, the available color information is severely limited by the small color bandwidth permitted.
Other types of analogue video signals which are particularly adapted to transmission by satellite and cable, and which lead to improved picture quality in comparison with existing standards, are presently being studied. These signals are based on a time multiplex of the three independent color components during the active line period of the scan line. Instead of coding the three components into one signal using the NTSC, PAL or SECAM standard, the components are sent sequentially using a time-compression technique. One version of this type of signal is know as MAC (Multiplexed Analogue Components). Signals generated by a time compression technique also adhere to the same basic format as the NTSC,
and SECAM standards, including the presence of a line-blanking interval and an active line period in each scan line. It should be noted that when a MAC signal is employed, digital data may also be transmitted during the line-blanking interval.
A MAC color television signal is illustrated in FIG. 3, which is an amplitude-vs.-time diagram of a single video line of 63.56 us duration. The first 10.9 us is in the horizontal blanking interval (HBI) 62, in which no picture information is transmitted. Following HBI 62 are chrominance signal 64 and luminance signal 66, either of which may be time-compressed. Between chrominance signal 64 and luminance signal 66 is a 0.28 us guard band 68, to assist in preventing interference between the two signals.
The MAC color television signal of FIG. 3 is obtained by generating conventional luminance and chrominance signals (as would be done to obtain a conventional NTSC or other composite color television signal) and then sampling and storing them separately. Luminance is sampled at a luminance sampling frequency and stored in a luminance store, while chrominance is sampled at a chrominance sampling frequency and stored in a chrominance store. The luminance or chrominance samples may then be compressed in time by writing them into the store at their individual sampling frequency and reading them from the store at a higher frequency. A multiplexer selects either the luminance store or the chrominance store, at the appropriate time during the active line period, for reading, thus creating the MAC signal of FIG. 3. If desired, audio samples may be transmitted during the HBI; these are multiplexed (and may be compressed) in the same manner as the video samples. The sample rate at which all samples occur in the multiplexed MAC signal is called the MAC sampling frequency.
With the adoption of a new transmission standard, a new and improved television service should offer a wider aspect ratio for, among other reasons, motion pictures have adopted wider aspect ratios. For example, motion pictures are commonly filmed with aspect ratios of 1.85:1. The Society of Motion Picture and Television Engineers (SMPTE) favors an aspect ratio for a television production standard of 16:9, which is the square of the standard 4:3 television aspect ratio. Another aspect ratio under consideration for new television systems is 5:3.
With the introduction of a widescreen television receiver, more samples per line of active video will occur in order to display the picture on the wider screen. Thus, the sampling rate of the picture elements will be higher if more samples are to be transmitted during the same active video line time. Correspondingly, the sample rate at the widescreen receiver would have to be higher.
One problem with the introduction of any new television system is its compatability with the standard 4:3 aspect ratio receivers presently in use by the public.
One way to achieve compatability is to transmit two television signals, one having the widescreen aspect ratio for receivers having a widescreen and the other having the standard aspect ratio for receivers having the standard screen. The standard aspect ratio television picture could be generated by selecting a portion of the widescreen picture. Both could be transmitted simultaneously for the simultaneous receipt at both aspect ratio televisions. The method of selecting a portion of the widescreen picture is known in the prior art. For example, U.S. Pat. No. 4,476,493, issued to Poetsch et al., and U.S. Pat. No. 4,223,343, issued to Belmares-Sarabia et al. both discuss this method of selecting a portion of a widescreen picture for display on standard televisions. This method, however, is costly for it requires dual storage and transmission of every picture.
Another possibility is to transmit the entire widescreen television signal and let the standard aspect ratio television skip alternate samples, allowing the widescreen picture to fit on the standard display. Such a method is described in U.S. Pat. No. 4,134,128, issued to Hurst. However, this method causes geometric distortion of the picture on the standard display.
Another possible method is to display the widescreen picture on the standard display, causing the widescreen picture width to be squeezed into the standard display and the height to be displayed by only a portion of the standard display height so as to affect a simulated widescreen aspect ratio. This method is contemplated in U.S. Pat. No. 4,394,690, issued to Kobayashi. This method, however, also geometrically distorts the picture, in addition to not making full use of the display screen.
Another problem with the introduction of any new television system is that the broadcasts or home recorded versions of 4:3 aspect ratio television signals would not be compatible with the new widescreen television receivers.